Valid evidence has been obtained to indicate that certain flavin oxido-reductase systems generate hydroxyl free radicals by virtue of their interaction with molecular oxygen during their normal redox activities. The phenomenon may involve an "electron leak" from these electron transferring systems. The data obtained indicate that these systems generate superoxide anions and hydrogen peroxide which then interact with each other and with inorganic iron to produce hydroxyl free radicals. The latter were shown to be capable of disrupting lysosomal membranes and other membranous organelles. The phenomenon may explain part or all of the requirement for dietary radical scavenging agents in animals.